It can be seen as a matter of economics that high deposition rates are highly desirable, since the higher the production is, the lower the unit cost becomes.
As a general rule, higher gold concentrations permit higher efficiency, current densities and plating rates.
However, for economic reasons (lower inventory, lower drag out, etc.) gold contents should be kept as low as possible.
It will be understood that higher current densities mean higher rates of deposition, since theoretically one ampere will deposit a definite amount of metal in one second. A further consideration is that the current efficiency, expressed as mg/ampere-minute, when reduced to very small values, renders the buildup of thick bright deposits difficult or impossible in high speed applications in which thick deposits have to be built up in a very short time, termed "retention time". That is, the low current efficiency works oppositely to the effect of high current density. Further, as stated in U.S. Pat. No. 4,436,595 at column 3, lines 25-29, the lower the temperature, the brighter the deposit, but the slower the plating speed, and vice versa; and as a compromise between brightness and plating speed, an operating temperature of 130.degree. F. is preferred. In fact, in practice, very few if any known acid gold plating baths give bright deposits at 150.degree. F., whereas, as will be seen in the ensuing description, the reverse is true for the baths of the present invention.
It is an object of the invention to improve plating efficiency at the low end of the range of current densities where high efficiency is generally not obtained, e.g., less than 20 ASF, preferably less than 10 ASF.
The use of nickel or cobalt chelates as brightener/hardeners is taught in U.S. Pat. Nos. 3,149,057 and '058. The use of aliphatic acids of 2 to 8 carbon atoms such as acetic, citric, tartaric, etc., when properly neutralized to act as buffers to maintain a pH between 3 and 5, is described.
U.S. Pat. No. 3,929,595, which is directed to employing a heterocyclic azohydrocarbon sulfonic acid or salt current extender, and a reduced amount of non-noble metal additions, also discloses the use of a weak organic acid, preferably citric or tartaric acid. It mentions that additional suitable weak organic acids include formic acid, lactic acid, kojic acid, itaconic acid, citraconic acid, gluconic acid, glutaric acid, glycolic acid, acetic acid and propionic acid.
U.S. Pat. Nos. 3,893,896 and 4,075,065 disclose alkali metal gold cyanide plating baths containing a metallic hardener such as cobalt citrate and nickel sulfamate, a Lewis acid such as boric acid, zirconium oxychloride and vanadyl sulphate, and a weak, stable aliphatic acid containing one or more carboxylic acid or hydroxy groups. It discloses as suitable organic acids, itaconic, citraconic, gluconic, glutaric, glycolic, citric, kojic, malic, succinic, lactic, tartaric and mixtures thereof.
U.S. Pat. No. 4,615,774 discloses a citrate-free bath for the electrodeposition of a gold alloy, which bath consists essentially of a bath soluble source of gold in an amount to provide a gold content of 4 to 50 g/l, a bath soluble source of nickel alloying metal in an amount to provide a nickel content of 0.5 to 20 g/l, oxalic acid in an amount of 20 to 100 g/l, and formic acid in an amount of 20 to 100 ml/l.
Also of interest is U.S. Ser. No. 912,171 filed Sept. 25, 1986 and now U.S. Pat. No. 4,670,107 incorporated herein by reference, which discloses a gold plating bath comprising an aqueous solution containing a soluble gold cyanide compound, a water soluble organophosphorous chelating agent, formic acid in a specified concentration, cobalt or nickel which may be introduced as their salts or chelates as brightener/hardeners and sufficient alkali to bring the pH to within a specified range.
As described therein, and as is also true in this application, the plating may be accomplished by any of the commercial means available such as barrel, rack and strip plating equipment and high speed continuous selective plating equipment. The products are useful for industrial purposes, especially for making electrical connections, e.g. as connectors. Depending on the type of equipment used, plating may be carried out at temperatures in the range of 90.degree. to 160.degree. F. and at current densities from about 0.5 to in excess of 1000 ASF.
One problem that arises in connection with said gold plating bath, is that the nickel-containing bath does not work as effectively at low current densities which prevail in barrel-type plating machines of which the Vibrobot, a vibratory unit, is a commercial example. The nickel percentage in the deposit is too high to meet certain specifications, i.e. MIL-G-45204C. For Type 1 deposits there should be 99.7% gold minimum; for Type 2 deposits 99.0% gold minimum. Also, the gold color is too light to be acceptable in connectors, i.e. too "brassy" for these applications. Further, the hardness is outside of acceptable specifications. It is too high to qualify. That is, it is above 200 Knoop so as not to meet grade C in the connector industry. It should be noted that reducing the nickel content in the bath does not reduce the hardness sufficiently; there is simply a loss of brightness. That is an unacceptable solution. There is a further difficulty when said bath is intended for use in a Vibrobot machine which arises from the fact that the machine has closed tanks plus the fact that it is customary in the art to use a replenisher. However, when employing formic acid, the replenisher is liquid and the conductivity medium is also liquid. Consequently the bath increases in volume. Therefore, there is a tendency to overflow. A new bath with solid replenisher and solid conductivity salts is desirable owing to the higher specific gravity. Of course, good solubility of the materials in the aqueous bath is necessary. The following table shows the physical properties of some dibasic acids.
TABLE __________________________________________________________________________ PHYSICAL CONSTANTS OF DIBASIC ACIDS Sol. M.P. g./100 g. Name Formula .degree.C. H.sub.2 O K.sub.1 K.sub.2 __________________________________________________________________________ Oxalic HOOC--COOH 189. 10.2.sup.20 5.7 .times. 10.sup.-2 6.9 .times. 10.sup.-5 Malonic HOOC--CH.sub.2 --COOH 135.6 139.4.sup.15 1.7 .times. 10.sup.-3 1.0 .times. 10.sup.-6 Succinic HOOC--(CH.sub.2).sub.2 COOH 185. 6.84.sup.20 6.65 .times. 10.sup.-5 2.3 .times. 10.sup.-6 Glutaric HOOC--(CH.sub.2).sub.3 COOH 97.5 83.3.sup.14 4.75 .times. 10.sup.-5 2.7 .times. 10.sup.-6 Adipic HOOC--(CH.sub.2).sub.4 COOH 151. 1.44.sup.15 3.76 .times. 10.sup.-5 2.4 .times. 10.sup.-6 Pimelic HOOC--(CH.sub.2).sub.5 COOH 103. 4.1.sup.20 3.48 .times. 10.sup.-5 3.23 .times. 10.sup.-6 Suberic HOOC--(CH.sub.2).sub.6 COOH 140. 0.142.sup.15.5 2.99 .times. 10.sup.-5 2.5 .times. 10.sup.-6 Azelaic HOOC--(CH.sub.2).sub.7 COOH 106.5 0.214.sup.20 2.96 .times. 10.sup.-5 2.7 .times. 10.sup.-6 Sebacic HOOC--(CH.sub.2).sub.8 COOH 134.5 0.10 2.34 .times. 10.sup.-5 2.6 .times. 10.sup.-6 __________________________________________________________________________
Accordingly it is a further object of the invention to provide an improved bath for electrodepositing gold containing a nickel or cobalt hardener which will be highly effective at low current densities, i.e. will operate at higher efficiency and give a more flexible product, especially with nickel, and which enables a solid replenisher to be used.